Comment on Structural Stability and Electronic Structure for Li3_3AlH6_6

Density functional calculations of the electronic structure are used to elucidate the bonding of Li$_3$AlH$_6$. It is found that this material is best described as ionic, and in particular that the [AlH$_6$]$^{3-}$ units are not reasonably viewed as substantially covalent

The influence of doping on electronic structure and stability of aluminum-hydride and lithium-tetrahydridealuminate(III)

У складу са интензивним развојем метода синтезе и модификације материјала за складиштење водоника, овде је представљена детаљна упоредна студија три AlH3 полиморфа (α-, β-, γ-) и LiAlH4 која има за циљ да објасни и усмери на потенцијална побољшања у циљу примене ових материјала. Прорачуни електронске структуре засновани на теорији функционала густине коришћени су за испитивање стабилности и природе веза у α-, β- и γ-AlH3. Електронска својства су проучавана коришћењем апроксимације генерализованог градијента (GGA- PBE), као и додатно Тран-Блахиним модификованим Бек-Џонсоновим потенцијалом (TB-mBJ) за измену. Истражeн је ефекат различитих металних допаната у α- и β-AlH3. Разматрана је супституциона инкорпорација металних допаната (Li, Sc, Ti, Cu, Cr, Fe, Nb, Mo, Zn или Zr) као и интерстицијално допирање са Li, Sc, Ti, Cu и Zr. Разматрана је кинетика десорпције водоника за неколико интерстицијално допираних случајева, израчунавањем стабилности сопствених тачкастих дефеката. Показано је да је супституционо/интерстицијално допирање алуминијум-хидрида Zr,Ti и Sc побољшало термодинамичке и кинетичке особине ових материјала у смислу могуће примене у системима за складиштење водоника. LiAlH4 је проучаван као комплексни хидрид и перспективан представник комплексних хидрида са великом теоријском гравиметријском густином од 10.6 wt%H2. У циљу снижавања температуре на којој се ослобађа водоник, проучаван је каталитички утицај Fe2O3 на LiAlH4 дехидрогенацију, као модел случај за разумевање ефеката који адитиви оксида прелазних метала имају у овом процесу. 57Fe - Месбауерова спектроскопија показала је промену у оксидационом стању гвожђа током десорпције водоника. Електронски трансфер из хидрида је разматран као предложени механизам дестабилизације LiAlH4 + 5 wt% Fe2O3.In line with the recent developments in the synthesis and modification of hydrogen storage materials, here presented a detailed comparative study of three AlH3 polymorphs (α-, β-, γ-) and LiAlH4 is aimed to explain and potentially guide the improvements in the applicability of these materials. Еlectronic structure calculations based on the density functional theory (DFT) are used to address stability and bonding in α-, β- and γ-AlH3. Electronic properties are studied using GGA-PBE for exchange-correlation, as well as the TB-mBJ potential for exchange. Attention is focused on an investigation of the effects of various metal dopants in α- and β-AlH3, to perceive a way of enhancing them. Substitutional incorporation of the metal dopants (Li, Sc, Ti, Cu, Cr, Fe, Nb, Mo, Zn, or Zr) is considered, as well as interstitial doping with Li, Sc, Ti, Cu, and Zr. The kinetics of hydrogen desorption is also considered for several interstitially doped cases, by calculating the stability of native point defects. Promising results are presented for Zr, Ti, and Sc-doped hydrides, showing how both their stability and kinetics of hydrogen desorption are improved. LiAlH4 is studied as a promising representative of complex metal hydrides, having a theoretical gravimetric density of 10.6 wt%H2. In order to decrease the temperature of hydrogen desorption, we investigated the catalytic influence of Fe2O3 on LiAlH4 dehydrogenation, as a model case for understanding the effects of transition metal oxide additives in this process. 57Fe Mössbauer study shows the change in the oxidational state of iron during hydrogen desorption. The electron transfer from hydrides is discussed as the proposed mechanism of destabilization of LiAlH4+5 wt%Fe2O

Complex Hydride Compounds with Enhanced Hydrogen Storage Capacity

The United Technologies Research Center (UTRC), in collaboration with major partners Albemarle Corporation (Albemarle) and the Savannah River National Laboratory (SRNL), conducted research to discover new hydride materials for the storage of hydrogen having on-board reversibility and a target gravimetric capacity of ≥ 7.5 weight percent (wt %). When integrated into a system with a reasonable efficiency of 60% (mass of hydride / total mass), this target material would produce a system gravimetric capacity of ≥ 4.5 wt %, consistent with the DOE 2007 target. The approach established for the project combined first principles modeling (FPM - UTRC) with multiple synthesis methods: Solid State Processing (SSP - UTRC), Solution Based Processing (SBP - Albemarle) and Molten State Processing (MSP - SRNL). In the search for novel compounds, each of these methods has advantages and disadvantages; by combining them, the potential for success was increased. During the project, UTRC refined its FPM framework which includes ground state (0 Kelvin) structural determinations, elevated temperature thermodynamic predictions and thermodynamic / phase diagram calculations. This modeling was used both to precede synthesis in a virtual search for new compounds and after initial synthesis to examine reaction details and options for modifications including co-reactant additions. The SSP synthesis method involved high energy ball milling which was simple, efficient for small batches and has proven effective for other storage material compositions. The SBP method produced very homogeneous chemical reactions, some of which cannot be performed via solid state routes, and would be the preferred approach for large scale production. The MSP technique is similar to the SSP method, but involves higher temperature and hydrogen pressure conditions to achieve greater species mobility. During the initial phases of the project, the focus was on higher order alanate complexes in the phase space between alkaline metal hydrides (AmH), Alkaline earth metal hydrides (AeH2), alane (AlH3), transition metal (Tm) hydrides (TmHz, where z=1-3) and molecular hydrogen (H2). The effort started first with variations of known alanates and subsequently extended the search to unknown compounds. In this stage, the FPM techniques were developed and validated on known alanate materials such as NaAlH4 and Na2LiAlH6. The coupled predictive methodologies were used to survey over 200 proposed phases in six quaternary spaces, formed from various combinations of Na, Li Mg and/or Ti with Al and H. A wide range of alanate compounds was examined using SSP having additions of Ti, Cr, Co, Ni and Fe. A number of compositions and reaction paths were identified having H weight fractions up to 5.6 wt %, but none meeting the 7.5 wt%H reversible goal. Similarly, MSP of alanates produced a number of interesting compounds and general conclusions regarding reaction behavior of mixtures during processing, but no alanate based candidates meeting the 7.5 wt% goal. A novel alanate, LiMg(AlH4)3, was synthesized using SBP that demonstrated a 7.0 wt% capacity with a desorption temperature of 150°C. The deuteride form was synthesized and characterized by the Institute for Energy (IFE) in Norway to determine its crystalline structure for related FPM studies. However, the reaction exhibited exothermicity and therefore was not reversible under acceptable hydrogen gas pressures for on-board recharging. After the extensive studies of alanates, the material class of emphasis was shifted to borohydrides. Through SBP, several ligand-stabilized Mg(BH4)2 complexes were synthesized. The Mg(BH4)2*2NH3 complex was found to change behavior with slightly different synthesis conditions and/or aging. One of the two mechanisms was an amine-borane (NH3BH3) like dissociation reaction which released up to 16 wt %H and more conservatively 9 wt%H when not including H2 released from the NH3. From FPM, the stability of the Mg(BH4)2*2NH3 compound was found to increase with the inclusion of NH3 groups in the inner-Mg coordination sphere, which in turn correlated with lowering the dimensionality of the Mg(BH4)2 network. Development of various Ak Tm-B-H compounds using SSP produced up to 12 wt% of H2 desorbed at temperatures of 400°C. However, the most active material can only be partially recharged to 2 wt% H2 at 220-300°C and 195 bar H2 pressure due to stable product formation. While gravimetric & volumetric targets are feasible, reversibility remains a persistent challenge

Discovery of Novel Complex Metal Hydrides for Hydrogen Storage through Molecular Modeling and Combinatorial Methods

UOP LLC, a Honeywell Company, Ford Motor Company, and Striatus, Inc., collaborated with Professor Craig Jensen of the University of Hawaii and Professor Vidvuds Ozolins of University of California, Los Angeles on a multi-year cost-shared program to discover novel complex metal hydrides for hydrogen storage. This innovative program combined sophisticated molecular modeling with high throughput combinatorial experiments to maximize the probability of identifying commercially relevant, economical hydrogen storage materials with broad application. A set of tools was developed to pursue the medium throughput (MT) and high throughput (HT) combinatorial exploratory investigation of novel complex metal hydrides for hydrogen storage. The assay programs consisted of monitoring hydrogen evolution as a function of temperature. This project also incorporated theoretical methods to help select candidate materials families for testing. The Virtual High Throughput Screening served as a virtual laboratory, calculating structures and their properties. First Principles calculations were applied to various systems to examine hydrogen storage reaction pathways and the associated thermodynamics. The experimental program began with the validation of the MT assay tool with NaAlH4/0.02 mole Ti, the state of the art hydrogen storage system given by decomposition of sodium alanate to sodium hydride, aluminum metal, and hydrogen. Once certified, a combinatorial 21-point study of the NaAlH4 â LiAlH4 âMg(AlH4)2 phase diagram was investigated with the MT assay. Stability proved to be a problem as many of the materials decomposed during synthesis, altering the expected assay results. This resulted in repeating the entire experiment with a mild milling approach, which only temporarily increased capacity. NaAlH4 was the best performer in both studies and no new mixed alanates were observed, a result consistent with the VHTS. Powder XRD suggested that the reverse reaction, the regeneration of the alanate from alkali hydride, Al and hydrogen, was hampering reversibility. The reverse reaction was then studied for the same phase diagram, starting with LiH, NaH, and MgH2, and Al. The study was extended to phase diagrams including KH and CaH2 as well. The observed hydrogen storage capacity in the Al hexahydrides was less than 4 wt. %, well short of DOE targets. The HT assay came on line and after certification with studies on NaAlH4, was first applied to the LiNH2 - LiBH4 - MgH2 phase diagram. The 60-point study elucidated trends within the system locating an optimum material of 0.6 LiNH2 â 0.3 MgH2 â 0.1 LiBH4 that stored about 4 wt. % H2 reversibly and operated below 220 °C. Also present was the phase Li4(NH2)3BH4, which had been discovered in the LiNH2 -LiBH4 system. This new ternary formulation performed much better than the well-known 2 LiNH2 â MgH2 system by 50 °C in the HT assay. The Li4(NH2)3BH4 is a low melting ionic liquid under our test conditions and facilitates the phase transformations required in the hydrogen storage reaction, which no longer relies on a higher energy solid state reaction pathway. Further study showed that the 0.6 LiNH2 â 0.3 MgH2 â 0.1 LiBH4 formulation was very stable with respect to ammonia and diborane desorption, the observed desorption was from hydrogen. This result could not have been anticipated and was made possible by the efficiency of HT combinatorial methods. Investigation of the analogous LiNH2 â LiBH4 â CaH2 phase diagram revealed new reversible hydrogen storage materials 0.625 LiBH4 + 0.375 CaH2 and 0.375 LiNH2 + 0.25 LiBH4 + 0.375 CaH2 operating at 1 wt. % reversible hydrogen below 175 °C. Powder x-ray diffraction revealed a new structure for the spent materials which had not been previously observed. While the storage capacity was not impressive, an important aspect is that it boron appears to participate in a low temperature reversible reaction. The last major area of study also focused on activating boron-based materials in order to exploit the tremendous gravimetric capacity of LiBH4. A number of LiNH2 â LiBH4 â transition metal (TM) systems were investigated for the following reasons. No additional leads were discovered in this system. Another major project activity was the assembly of a high throughput synthesis system. The automated synthesizer was set up in a glovebox and was capable of handling liquids and powders and carrying out sealed block syntheses up to 250 °C. Unfortunately, the synthesizer could not handle the delivery of the fine powders required fro hydrogen storage applications. Although the powder delivery system was overhauled and redesigned several times, this problem was never remedied

Metal supported carbon nanostructures for hydrogen storage

Carbon nanocones are the fifth equilibrium structure of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this thesis was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. X ray diffraction (XRD) technique confirmed the complete metal reduction and transmission electron microscope (TEM) analysis showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt% were achieved and high dispersions were retained for loadings as high as 15 wt%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed. In comparison the same route of pre-treatment and metal impregnation has been done over graphite nanofibres (GNF) and the hydrogen uptake showed an adsorption superior of the cumulative contribution of the substrate and metal catalyst attributing this to hydrogen spillover. The GNF have been impregnated also with another metal catalyst Ni showing as well the phenomenon of hydrogen spillover. The attempt to impregnate the carbon nanocones with a mixture of Pd-Ni, Pd-Cu and Pd-Ag resulted in an increase of hydrogen uptake for the first two but a decrease for the last of these. The carbon nanocones have been also impregnated with a Mg organometallic precursor dibutyl magnesium (DBM) and then decomposed without the use of hydrogen environment synthesizing successfully MgH2. The stoichiometry and the enthalpy of this decomposition have been studied. Furthermore, the DBM has been mixed with another hydride LiALH4 and the decomposition reaction of the complex hydride has been studied

Metal supported carbon nanostructures for hydrogen storage

Carbon nanocones are the fifth equilibrium structure of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this thesis was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. X ray diffraction (XRD) technique confirmed the complete metal reduction and transmission electron microscope (TEM) analysis showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt% were achieved and high dispersions were retained for loadings as high as 15 wt%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed. In comparison the same route of pre-treatment and metal impregnation has been done over graphite nanofibres (GNF) and the hydrogen uptake showed an adsorption superior of the cumulative contribution of the substrate and metal catalyst attributing this to hydrogen spillover. The GNF have been impregnated also with another metal catalyst Ni showing as well the phenomenon of hydrogen spillover. The attempt to impregnate the carbon nanocones with a mixture of Pd-Ni, Pd-Cu and Pd-Ag resulted in an increase of hydrogen uptake for the first two but a decrease for the last of these. The carbon nanocones have been also impregnated with a Mg organometallic precursor dibutyl magnesium (DBM) and then decomposed without the use of hydrogen environment synthesizing successfully MgH2. The stoichiometry and the enthalpy of this decomposition have been studied. Furthermore, the DBM has been mixed with another hydride LiALH4 and the decomposition reaction of the complex hydride has been studied

Hydrogen storage in complex hydrides: past activities and new trends

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